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This plug-in is complicated by the fact that it uses a mixture
of cached and uncached lists.
Even if a list is cached, the list is not read from the database until
required.  I.e. it is lazily read.  But once read a cached list is maintained
in a Collection object for as long as the parent object exists.
<p>
A weak map is maintained of all objects to ensure that only a single instance
of a given object is constructed.
<p>
This plug-in caches everthing except
the list of transactions in the session object.  Note that it is the list, not
the object class, that is cached or uncached.  Though as the only list of transactions
is in the session, this distinction is academic.
<p>
Entries are cached, but only while
the transaction is in a materialized state.
I.e. The entries are
first read when the transaction
is materialized AND the entries are iterated.  The list is maintained in
memory from then on.  If there are no references left to the transaction
then the entire transaction, including the entry list, is garbage collected
and is removed from the weak reference map.  As all updates (including new or deleted entries) to committed to
the database when made, the transaction can be re-materialized correctly.
<p>
If a list is uncached then a query will be sent to the database each time
the list is iterated.  The weak reference map only prevents us from instantiating
duplicate objects but does not save any database access as the required data has
already been fetched in the query result set.
<p>
An uncached list MUST look to the weak reference map before materializing
each object.  A cached list keeps a list of all objects in the list (and therefore
we do not need to look at the weak reference map, although all the objects
will be in the map).
<p>
Scanning the transactions for entries that match a given criteria is not going
to be the most efficient.  The reason is that first a fetch of all transactions
is required, then a fetch of all entries in that transaction.  It may be more efficient
to fetch both in a join, but to keep things simple this is not done.  A special method is provided for fetching all entries
in a given account.  Other such methods may need to be added.
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